Compared to Nb3Sn and other metal superconductor materials, Bi-based and Y-based oxide superconductor materials have a higher critical temperature (Tc), so applications as wires for electromagnets and power transmission are greatly anticipated.
Bi-based oxide superconductor materials are already in the commercialization stage (see Japanese Patent Publication (A) No. 3-138820), but when applying a magnetic field in parallel to the c-axis, the irreversible magnetic field (Birr) at 77K (cooling temperature by liquid nitrogen) is a low 0.5 T or less, so even when formed into wire, the applications end up being limited.
On the other hand, compared with Bi-based oxide superconductor materials, a superconductor material mainly comprised of an REBa2Cu3O7-δ-based oxide (hereinafter referred to as a “RE123-based oxide superconductor”) has a higher critical current density (Jc) and irreversible magnetic field (Birr), so application as a wire for a strong magnetic field and a strand for cable for transmission of high voltages are greatly anticipated.
However, in the case of an RE123-based oxide superconductor material, oriented crystals can be obtained by the (i) melt growth method, but the heat treatment temperature is a high 1000° C. or more. The Ag sheath materials conventionally used for fabrication of wire (melting point of Ag: approximately 960° C.) cannot be used. Further, with (ii) rolling or other mechanical techniques, the crystals do not orient, the grain boundary bonds are weak, a high current density cannot be obtained, etc., so even if making a wire using an Ag sheath material by the PIT method (powder in tube method), the desired current characteristics could not be obtained (see Jpn. J. Appl. Phys., Vol. 26, No. 5 (1987) pp. L865 to L866).
Consequently, as methods for forming a wire using a metal base material, the coating and heat decomposition method, physical vapor deposition method, etc. providing an orienting intermediate layer on the metal base material and forming an orienting superconductive film over that have been developed (see Japanese Patent Application No. 11-504767 [Japanese Patent Publication (A) No. 2002-505032] and Adv. Superconductivity VI (1994) pp. 749-754). However, these methods are slow in the film forming rate and therefore have problems in terms of mass productivity.
In this way, in the development of long RE123-based oxide superconducting wire stably providing superior superconducting characteristics (high critical current density and high irreversible magnetic field) or RE123-based oxide superconductors forming the basic material for wires, there are still many problems which remain to be solved, but the development of the above RE123-based oxide superconducting wire would have extremely great merits not only from the viewpoint of utilization as wire for a strong magnetic field or strands of a cable for high voltage transmission, but also from the viewpoint of resource and energy saving. Presently, Japan, the U.S., and Europe are fiercely competing for the development of technology for improving superconducting characteristics and mass producing RE123-based oxide superconducting wire able to easily handle even multi-core applications.